During axle alignment, the toe and camber angles of the vehicle are determined and set as necessary. An additional subtask of axle alignment is to check the steering geometry. This is carried out by measuring the movement of a wheel during a steering movement. The spread angle and the caster angle are typically measured in this process.
The characteristics of the steering geometry depend on the mechanical configuration of the steering. A differentiation is made between classic geometries having a fixed steering axis and so-called multilink suspensions. Characteristics for describing the steering geometry with a fixed steering axis, such as spread angle, caster angle, caster offset, and kingpin offset, are defined in DIN 70000.
To determine the characteristics, the toe angle and the camber angle are typically measured during a steering movement (“steering wheel turn routine”). The variables spread angle and caster angle are then calculated from the toe and camber values of the wheel at different steering angles.
In “HUNTER ENGINEERING CO, BRIDGETON, Mo., USA: “STEERING GEOMETRY AND CASTER MEASUREMENT”, SAE TECHNICAL PAPER SERIES, no. 850219, 1985, pages 1-10, XP008068693 WARRENDALE, Pa., USA”, Daniel B. January discusses the calculation of the spread angle and the caster angle from two individual measurements carried out during a wheel steering movement. Since only two individual measurements are carried out, measurement errors in the two toe and camber measurements result in correspondingly significant errors in the steering geometry calculation.
Patent document EP 1 739 390 B1 discusses the linking of more than two toe and camber measurements to increase the measurement accuracy by fitting a polynomial to the measurement curves of the toe and camber angles.
The toe and camber measurement may be carried out using different known axle alignment methods. Newer axle alignment methods allow optical measurement of the axle geometry via 3D recording of the wheel movement. In these methods, the geometric 3D position and alignment of the axis of rotation and the center of rotation of the wheels are determined. This may be carried out by measuring so-called targets with optical target marks on the wheel (refer, for example, to DE 197 57 763 A1, EP 1 042 643 B1) or via three-dimensional recording of the surface of the wheel, e.g., by projecting a pattern onto the wheel surface (refer, for example, to EP 187 51 64 B1, WO 2008/046715 A1).